It is well known that copolymers of vinylidene fluoride and trifluoroethylene are employed and are being developed for use in electric/electronic devices (e.g. transducers, sensors, actuators, ferroelectric memories, capacitors) because of their ferroelectric, piezoelectric, pyroelectric and dielectric behaviour/properties, piezoelectric behaviour being particularly used.
As is well known, the term piezoelectric means the ability of a material to exchange electrical for mechanical energy and vice versa and the electromechanical response is believed to be essentially associated with dimensional changes during deformation or pressure oscillation. The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied).
Ferroelectricity is the property of a material whereby this latter exhibits a spontaneous electric polarization, the direction of which can be switched between equivalent states by the application of an external electric field
Pyroelectricity is the ability of certain materials to generate an electrical potential upon heating or cooling. Actually, as a result of this change in temperature, positive and negative charges move to opposite ends through migration (i.e. the material becomes polarized) and hence, an electrical potential is established.
It is generally understood that piezo-, pyro-, ferro-electricity in copolymers of vinylidene fluoride and trifluoroethylene is related to a particular crystalline habit, so called beta-phase, wherein hydrogen and fluorine atoms are arranged to give maximum dipole moment per unit cell.
Vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymers are typically provided as semicrystalline copolymers which can be shaped or formed into semicrystalline, essentially unoriented and unstretched, thermoplastic film or sheet or tubular-constructed product via such well known processing methods as extrusion, injection moulding, compression moulding and solvent casting.
In addition to the good piezo-, pyro-, ferro-, and di-electric properties said copolymers have several preferable properties over ceramic materials such that they can be easily formed in thin films having a large surface area and they can be mass-produced.
Said VDF-TrFE copolymers are well known in the art and are notably described in U.S. Pat. No. 4,778,867 (PRIES SEYMOUR (US)) Oct. 18, 1988, U.S. Pat. No. 4,708,989 (THOMSON CSF (FR)) Nov. 24, 1987, U.S. Pat. No. 4,784,915 (KUREHA CHEMICAL IND CO LTD (JP)) Nov. 15, 1988, U.S. Pat. No. 4,173,033 (DAIKIN IND LTD (JP)) Oct. 30, 1979.
However, VDF-TrFE copolymers of the prior art suffer from a poor elastic behaviour; actually, in operations of piezoelectric devices, the piezoelectric materials are submitted to sequential strains and deformations: the inherent flexibility or deformability of the materials (or in other words their elastic behaviour) is thus a key parameter for ling-life operations.
On the other side, optimization of piezoelectric, pyroelectric or ferroelectric effect requires crystalline phase beta fraction, as above described, to be maximized, crystallinity being otherwise related to brittle behaviour of the material.
There is thus still a need in the art for VDF-TrFE copolymer materials, and a method for their manufacture, which can fulfil such antagonist requirements and which offer increased flexibility and deformability, while maintaining outstanding piezoelectric, ferroelectric pyroelectric, and/or dielectric properties.
The invention described herein provides a material and a method which satisfy these needs.